Note: the last CommitPoint is the most recent one,
i.e. the "front index state". Be careful not to delete it,
unless you know for sure what you are doing, and unless
you can afford to lose the index content while doing that.

onInit

This is called once when a writer is first
instantiated to give the policy a chance to remove old
commit points.

The writer locates all index commits present in the
index directory and calls this method. The policy may
choose to delete some of the commit points, doing so by
calling method delete()
of IndexCommit.

Note: the last CommitPoint is the most recent one,
i.e. the "front index state". Be careful not to delete it,
unless you know for sure what you are doing, and unless
you can afford to lose the index content while doing that.